1. Field of the Invention
The present invention relates to coating compositions for polyester resins.
2. Description of the Prior Art
Polyester resin films and particularly polyethylene terephthalate films have excellent properties such as good physical strength, good heat resistance, good gas-impermeability and good electrical properties, etc. However, polyester resin films have defects in that they are not heat-sealable and they are difficult to write or print on.
Hitherto, a certain synthetic resin layer has been provided on polyester resin films in order to remove these defects. For example, a vinylidene type polymer layer is formed on a polyester resin film to improve the gas-impermeability and to render the film heat sealable, whereby the film can be used for heat sterilization packing or vacuum packing of foods. Further, a polyester resin film having a layer containing a matting agent or a bulking agent therein is used as a tracing film or a scribing film. However, in the case of providing a synthetic resin layer on the polyester resin film, an activation treatment such as a corona discharge treatment, ultraviolet light exposure or a flame treatment, etc. of a surface of the base film is applied in order to obtain firm adhesion in addition to the formation of a subbing layer thereon. However, these treatments are not very effective for polyester resin films. Furthermore, a method which comprises dipping the polyester resin films in an aqueous acid or alkali solution and a method which comprises dipping the polyester resin films in trichloroacetic acid or phenols have been considered. However, these methods are not suitable, because the handling of chemicals is dangerous because of their toxicity, and after the treatment the treating solution remaining in the base film due to permeation excludes as time passes adversely influencing the synthetic resin coating layer or injuring the appearance of the product. Accordingly, at present, an adhesive layer, a socalled subbing layer, is usually provided on a polyester film where such is to be coated with a synthetic resin layer.